The present invention relates to a double phase lock loop arrangement which has particular, but not inclusive, application in portable radio receiving apparatus or in a portable transceiver.
When designing portable radio receivers, particularly narrow band multi-channel receivers, it is useful to employ automatic frequency conrol (AFC). This gives the advantage of maintaining the intermediate frequency (IF) signal in the center of the IF filter pass-band when the local oscillator frequency drifts due to temperature changes or component aging. Also the distortion produced in the receiver is minimized and matching of the cystal filter becomes less critical.
However, in multi-channel receivers, the automatic frequency control circuit may be affected by strong signals in channels adjacent to the selected channel. This particular applies in a receiver which works on the principle of a dual phase lock loop. Such a receiver is described in a paper read by Stephen W. Watkinson at the Communications 74 conference held in Brighton, England, during July 1974 and published in the Conference Proceedings at pages 13.1/1 to 13.1/8, the contents of which are hereby incorporated by reference. In this system a very high level adjacent channel signal will produce a beat frequency with the intermediate frequency voltage controlled crystal oscillator (IFVCXO). This waveform will frequency modulate the IFVCXO producing a non-sinusoidal error waveform containing a D.C. component. The D.C. component will be fed to the local oscillator causing its frequency to be pulled towards the frequency of the adjacent channel signal thus reducing the rejection of the adjacent channel signal. If the level of the adjacent signal is now reduced, the pulling effect may be maintained typically down to a level of 30 dB below that at which it started.
This problem may be reduced by including a low-pass filter in the feed-back path to the IFVCXO, but if the filter has rapid rolloff, the loop may become unstable. In practice, the loop filter must be limited to a single pole network and, consequently for a 12.5 KHz channel spacing, the attenuation of the 12.5 KHz best frequency is not sufficient to overcome the problem. An alternative arrangement would be to include a notch filter such as a Twin-Tee circuit, with the notch frequency at the difference between the selected channel and adjacent channel frequencies. However, if the notch is too deep, loop instability will again to produced because of reactive components in the notch filter circuit.